Orinda Formation

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Orinda Formation
Stratigraphic range: Pliocene
TypeGeologic formation
UnderliesMoraga Formation
OverliesTice Shale, of
Monterey Formation Group
Primarycoarse conglomerates
RegionAlameda County and
Contra Costa County,
CountryUnited States
Type section
Named forOrinda, California

The Orinda Formation is a Pliocene epoch geologic formation in the Berkeley Hills of the East Bay region of the San Francisco Bay Area, California.[1]

It is found within Alameda County and Contra Costa County.[1][2]


The Orinda Formation is a coarse alluvial conglomerates sedimentary formation. It underlies the volcanic Moraga Formation.[3]

It preserves fossils dating back to the Pliocene epoch of the Neogene period.[4]

Geologic Hazards[edit]

The Orinda Formation is prone to landsliding due to intrinsic properties such as its weak cohesive strength and low friction angle. Landslides less than or equal to 10 acres in size occur as slides, slumps, and earthflows on dip or parallel to dip slopes. In the formation, landslides greater than or equal to 10 acres in size are common to very common and occur as slumps and slides on anti-dip slopes.

See also[edit]


  1. ^ a b USGS.gov: "Upper Cretaceous and Lower Tertiary Rocks Berkeley and San Leandro Hills, California", by J. E. Case, GEOLOGICAL SURVEY BULLETIN 1251-J, 1968.
  2. ^ Geology Blog: "Grizzly Peak and Moraga basalt"
  3. ^ Lawrence Berkeley Lab.gov: "Berkeley Lab Geologist Studies the Ground Beneath His Feet"
  4. ^ Various Contributors to the Paleobiology Database. "Fossilworks: Gateway to the Paleobiology Database". Archived from the original on 31 July 2014. Retrieved 8 July 2014.